1. Field of the Invention
The present invention relates to a piezoelectric actuator and liquid discharge apparatus having a piezoelectric device and a driver for driving the piezoelectric device, and a power generating apparatus having a piezoelectric device.
2. Description of the Related Art
Piezoelectric materials can mutually convert the electrical energy and mechanical energy by their own intrinsic functions (properties). As having excellent properties in the electromechanical mutual conversion and as mechanical vibrating bodies, piezoelectric materials are widely applied to frequency control, such as frequency filters and the like, and electronics components for telecommunication. Further, in the field of mechatronics, the electromechanical mutual conversion function of piezoelectric materials has gained recognition along with the integration and miniaturing technologies.
Generally, a piezoelectric strain constant (piezoelectric d-constant) and a voltage output constant (piezoelectric g-constant) are used as indices for indicating the performance of piezoelectric materials (piezoelectric performance). The d-constant is an index representing the amount of strain per unit electric field (transmission capability), and a higher d-constant indicates a higher performance as an actuator used in an inkjet recording head and the like. The g-constant is an index representing a field intensity generated per unit stress (reception capability), and a higher g-constant indicates a higher performance as a sensor.
In general electric field induced strain in which, when the vector component of spontaneous polarization axis corresponds to electric field application direction, expansion or contraction occurs according to increase or decrease in the strength of applied electric field, the following holds true. That is, d=k√ε0√ε·√s, g=d/ε0ε (k is the electromechanical coupling coefficient, εo is the vacuum permittivity, ε is the relative permittivity, and s is the elastic compliance). Thus, in the general electric field induced strain, either of the two constants becomes greater for a higher electromechanical coupling coefficient. Therefore, in the field of either actuators or sensors, piezoelectric materials having high electromechanical coupling coefficients are sought.
As an attempt to increase electromechanical coupling coefficient, a literature by K. Nakamae et al., “Development of High Resolution and Wide Band Piezoelectric Composite for Ultrasonic Probe”, SEI Technical Review, September, Vol. 163, pp. 48-52, 2003 (Non-patent Document 1) describes a composite piezoelectric material for ultrasonic sensors having a composite structure of a polymer and a plurality of bar-like bulk bodies of lead zirconate titanate (PZT) disposed in the polymer in parallel with each other to alleviate the mechanical constraints by the flexibility of the resin, thereby effectively extracting vibrations in the thickness direction. In the method of producing a composite structure, as in Non-patent Document 1, the manufacturing process becomes complicated and the piezoelectric performance may possibly be degraded due to the polymer portion. Thus, it is preferable that a piezoelectric material itself has a high electromechanical coupling coefficient.
A literature by M. Matsushita et al., “Development of Large Diameter Piezo-single Crystal PMN-PT of High Energy Transfer Efficiency”, JFE Technical Report, No. 8, pp. 43-48, June, 2005 (Non-patent Document 2) has succeeded in increasing the electromechanical coupling coefficient through bulk single crystallization of a solid solution of Pb(Zn,Nb)O3 and PbTiO3.
Application of materials having both a high d-constant and a high g-constant to ultrasonic sensors and power generating devices are also expected. In ultrasonic sensors, a piezoelectric material having well-balanced transmission capability and reception capability is preferable, while in power generating devices, the product of d-constant and g-constant of a piezoelectric material serves as the performance index. The d-constant takes a large value for a high permittivity while d-constant takes a small value for a high permittivity, as shown in the formulae above, Consequently, the material (PZT-PT) of Non-Patent Document 2 having both a high electromechanical coupling coefficient and a high permittivity may provide a high value for the d-constant but not for the g-constant.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a piezoelectric device having excellent transmission and reception capabilities, and is favorably used as an ultrasonic sensor or a power generating device, as well as a piezoelectric actuator or a sensor. It is a further object of the present invention to provide a piezoelectric actuator, a liquid discharge apparatus, and a power generating apparatus, each having the piezoelectric device described above.